The knee is generally regarded as being one of the most inherently unstable joints in the human body due in part to the complex, interrelated motions to which the knee is subjected during normal motion. In the past, various mechanical approaches have been proposed in an attempt to approximate the natural action of the knee through artificial total joints However, advanced conditions of disease or serious traumatic injury to the knee joint complicate surgical repair and efforst to simulate the natural knee motion through the use of a prosthesis.
Under circumstances where the knee is beyond acceptable repair using conventional total joint reconstruction with separated condylar type or posterior stabilized prostheses as the articular femoral and tibial surfaces, it has become acceptable practice to reconstruct the knee with a hinged type prosthesis. For example, U.S. Pat. No. 4,262,368 in the name of Lacey, issued Apr. 21, 1981 and assigned to the assignee of the subject invention, discloses a prosthesis for replacement of the human knee joint having articulating femoral and tibial components hingedly connected about a transverse, or medial-lateral axis. The Lacey prosthesis provides appropriate constraint while allowing for rotation and distraction during knee motion. This minimizes prosthetic and/or fixation failure by reducing torsional and tensile stresses The Lacey prosthesis also includes a patellar track formed in the femoral component as a central concave groove. The patellar track provides a contact surface for proximal-distal movement and positioning of the patella, either natural patella or prosthetic patella, during knee flexure.
The primary disadvantage of this Lacey prosthesis is manifest when the natural patella remains in the knee joint after surgical implantation of the knee prosthesis. The natural patella is salvaged in a significant number of pediatric applications. As shown in FIG. 1, when the knee is flexed at or near its maximum, there is a possibility that the patellar track may exceed by relative movement, i.e., overrun, the patella. In these situations, it is possible that the superior edge of the patella will become snagged, or lodged, in the hinge joint below the patellar track. Under these circumstances, an extremely painful locked knee condition occurs.